Old age is always faintly unnerving. Although, at 88, Alain Resnais isn't by any means the most venerable of active film-makers, it's still hard to credit that the film I've come to Paris to talk to him about – Wild Grass, an authentic surrealist romance, as far from being geriatric in style as it's possible to imagine – was made by this elegant, eloquent gentleman sitting opposite me at the Hôtel Claridge, near the Champs Elysées.

I last met Resnais a couple of decades ago, and he has remained much as I remembered: the superb mane of snow-white hair, flaming red shirt, tightly knotted black tie and trademark white trainers. All that's missing is a viewfinder dangling on his pullover, as nonchalantly as a monocle.

Even his legendary modesty is intact. On none of his films will you ever read the credit "Un film d'Alain Resnais". "How could I insist on so pompous a credit for Wild Grass?" he says. "You've seen it. It's almost as much Eric Gautier's film as my own." (Gautier is his cinematographer; it is his Fauvist colour schemes and comic strip-inspired lighting effects that render Wild Grass so distinctive.) Then, uniquely in my experience of famously modest artists, he contrives to make light even of his modesty. "If I'd written the film as well as directing it, I might well have thought of calling it 'Un film d'Alain Resnais'."

For some buffs, admiration of Resnais's work has remained jammed at his early trio of masterpieces: Hiroshima Mon Amour; Last Year in Marienbad; and Muriel. In the 1960s, his reputation was that of a "difficult", elitist film-maker who, spurning professional screenwriters and experienced studio musicians, chose instead to collaborate with literary writers (Marguerite Duras, Alain Robbe-Grillet, Jorge Semprún) and classical composers (Hanns Eisler, Hans Werner Henze). As for his subject matter, it was unrepentantly serious: reducible, for many a lazy reviewer, to those vaudeville duos, time and memory (Hiroshima), and appearance and reality (Marienbad), brought out of mothballs, like Neil Simon's Sunshine Boys, for one last benefit.

Resnais denies that he was ever so airlessly single-minded an intellectual. "I doubt," he says, "I was ever an intellectual at all." And certainly, in recent years, his films have lightened up to a degree unique in the career of any major director. He has made a musical, On Connaît la Chanson (Same Old Song, dedicated to Dennis Potter); an adaptation of the 1920s operetta, Pas Sur la Bouche (Not on the Lips); a pair of comedies based on Alan Ayckbourn plays, Smoking/No Smoking and Private Fears in Public Places; and now, with Wild Grass, a film whose riotous visuals draw their inspiration from Will Eisner's neo-baroque comic strip The Spirit.

I ask if this gradual change of tone represents what Freudians used to call "the return of the repressed" – if, in short, he'd always had a secret craving to make public his private fondness for popular culture. "You've got to remember," he says, "that the cinema of the 1950s specialised all but exclusively in escapist entertainment. And we, I mean my generation of French film-makers, sought precisely to escape from that escapism. We were young and ambitious and we wanted to address the big issues from which the cinema preferred to avert its eyes – in my own case, the Holocaust, the atomic bomb, the Algerian war. Now, in France, 230 films are released every year and I would say that fully 60% consciously set out to expose some social or political abuse. It's become almost the norm. Well, I dislike norms. Blissfully liberated from the pressure to compete, I'm free to play with what Orson Welles called the biggest electric train set in the world."

Sneaking into Scarborough

It's from his penchant for sometimes near-prankish experimentation that his enthusiasm for Ayckbourn derives. "I've always adored the theatre, as much as the cinema, and I've also always been an anglophile. So I used to subscribe to [defunct magazine] Plays & Players, and it was there I first read about this mysterious English playwright who subjected traditional theatrical conventions to audacious squeezings and stretchings. I would read how he would have two separate decors share the same stage simultaneously, or set the same story in three different locations."

These devices, however, proved frustratingly tricky to visualise, and so he decided to go and see for himself. With Sabine Azéma, his former companion and a founding member of his films' repertory company, he turned up in Scarborough, not really knowing what to expect. "We saw one of Alan's plays – I no longer recall which. We were utterly bowled over by its inventiveness, and I started going year after year until it became a sort of pilgrimage. Then one year, one of the actors took Alan aside and said, 'You see that white-haired man in the second row? I think it's the French film director Alain Resnais.' Alan laughed, but he was sufficiently intrigued to approach me. And that's how I ended up making Smoking/No Smoking and Private Fears in Public Places."

Why hadn't he introduced himself before? Resnais grins, a grin so wide it seems to stretch beyond the contours of his face. "Funny, that's exactly what Alan asked!" he says. But why? "Oh, I would never do that."

The premise of Wild Grass, based on a book by Christian Gailly, is vanishingly slim. Marguerite, an almost nerve-wrackingly chic dentist (a jittery, frizzy-haired Azéma), has her wallet snatched in the street. It's recovered, minus cash, by Georges (André Dussollier, another Resnais regular, here both genial and sinister). Once he's made contact with her, they embark on a relationship ("affair" is most definitely not the word) that imperceptibly develops into one of the purest expressions, at least since the heyday of Luis Buñuel, of cinematic amour fou. It's a film of spectacular direction – not production – values.

I should add that its climactic line of dialogue – "Mummy, when I'm a cat, will I be able to eat cat munchies?" – delivered by a little girl we haven't set eyes on prior to that moment, has prompted a lot of head-scratching. "Everything in the film," he says, "is scrupulously faithful to Gailly's novel." Then he adds, without any prompting: "Including that last line, which has prompted so much speculation. But because it's a film, not a novel, of course the impact is going to be very different."

Ultimately, it's that perverse, slightly dated fidelity to his source material (rather refreshing, like a lot of dated things) that distinguishes Resnais's work from the films of those who trade in what have become cliches of the imagination. Take comic strips. When, from one image to the next, the background switches from yellow to blue to black, the reader accepts it as a generic convention. When Resnais pulls an identical stunt in Wild Grass, the effect is deliciously disorienting.

"I enjoy surprising as a film-maker, just as I enjoy being surprised as a spectator. And, you know, the director is the first spectator of his film. When Gautier and I decided that Will Eisner's graphic novels would be a determinant factor in the 'look' of Wild Grass, neither of us really had any idea what the result would be. It was only when I viewed the rushes that I knew what kind of a film we were making."

One last question. Would he ever shoot a realistic film? "It's not that I wouldn't, it's that I couldn't," he says, modest to the end.

The iconic French film-maker has died aged 91, a month after the premiere of his last film. Peter Bradshaw looks back over a career marked by intellectual engagement and impish humour, and whose highlights included Night and Fog , Hiroshima Mon Amour and Last
Year In Marienbad